The invention relates to the field of mains power supply units for supplying electronic assemblies, in particular the invention relates to a capacitor power supply unit comprising a charging capacitor for charging up a further capacitor (storage capacitor) for the purpose of operating an electric power consumer, a rectifier which is disposed between the charging capacitor and the storage capacitor, and a switching means which by applying circuit engineering technology is disposed between the charging capacitor and the storage capacitor and has a control input which has applied to it the outputs of a member which determines the point in time, at which switching is to occur, which member is influenced by a small reference voltage in comparison with the mains alternating voltage, for the purpose of determining the point in time for switching over the switching means from a switch position which charges the storage capacitor, to a switch position which interrupts the charging procedure, in dependence upon the voltage applied on the output side to the charging capacitor, and a charging state-determining member for the purpose of determining the charging state of the storage capacitor. The invention also relates to a method of operating a capacitor power supply unit.
Capacitor power supply units are widely used for the purpose of generating auxiliary voltage supplies which are not separated from the mains supply. These types of auxiliary voltages usually serve to supply electronic assemblies, for example, to supply the electronics used for controlling the rotational speed in boring machines. The principle of a capacitor power supply unit is based upon the cyclic recharging of the charging capacitor which is used as a charging pump for the storage capacitor and charges same during each mains supply cycle. A breakdown diode is used for the purpose of stabilizing this type of capacitor power supply unit. In the context of these statements, the term breakdown diode relates to all diodes which are reverse biassed and are also known as a Zener diode or avalanche diode.
By reason of their straightforward and cost-effective design, capacitor power supply units are widely used for small output powers with a virtually constant load. However, a problem arises in the use of this type of capacitor power supply unit, if the load is relatively large and is subjected to severe fluctuations in dependence upon the operating point. This change in load can lead to an extreme loading of the breakdown diode, which ultimately can cause the breakdown diode, the storage capacitor and thus the entire power supply unit to be destroyed. This effect is particularly damaging if this type of capacitor power supply unit is encapsulated, for example, in a housing, so that the heat produced in the breakdown diode cannot be dissipated or is only dissipated inadequately, so that the thermal loading can cause the breakdown diode to become unsoldered. The capacitor power supply unit is then no longer functional.
In order to lessen these disadvantages, capacitor power supply units have been developed, as described, for example, in DE 38 01 399 A1, wherein the switching procedure for switching over the switching means from a switch position which charges the storage capacitor, to a switch position which interrupts the charging procedure, is performed at a point in time, at which the switching means is supplied with only a small amount of current. As a consequence, no steep switching transients should be produced in the case of this switching procedure. This is achieved by virtue of the fact that the switching means has a control device connected upstream, which serves to move the switching means to its switch position which interrupts the charging procedure if the voltage applied on the output-side to the charging capacitor is small corresponding the cycle of the mains voltage. At such a point in time when switching occurs, the switching means is only supplied with a small amount of current. This capacitor power supply unit is then switched to the charging operation, if the output voltage of the storage capacitor is less than a predetermined desired value. This switching procedure is exclusively dependent* upon the charging state of the storage capacitor and can therefore be performed at any time with regard to the cycle of the mains voltage. Even if, in the most favourable case, the switch can only be subjected to low voltage at this point in time when switching occurs, the switching means is still supplied with a relatively high level of current, so that the occurrence of current peaks cannot be excluded.
When the electric power consumer is operated in a continuous manner, the required current is drawn off continuously from the storage capacitor. By virtue of the procedure of controlling the switching means, as described in this document, the switching means is continuously switched in a reciprocating manner between the two operating positions corresponding to the frequency of the mains supply cycle or the rectified voltage half-waves. The loss of power which occurs in particular when switching the switching means to the charging operation of the storage capacitor means that there is the risk that the electronic components can be heated up to the point of being destroyed.
On the basis of the discussed prior art, the object of the invention is therefore to provide a capacitor power supply unit which is also suitable for permanently operating the electric power consumer connected thereto.
Furthermore, the object of the invention is to provide a corresponding method of operating a capacitor power supply unit.
In accordance with the invention, the object relating to the device is achieved by virtue of the fact that the switching time-determining member and the charging state-determining member, when connected together by wires so as to represent a logical AND-function, influence the control input of the switching means, so that both switching procedures of the switching meansxe2x80x94switching the capacitor power supply unit to the charging operation and switching same for the purpose of interrupting the charging operationxe2x80x94are performed at a point in time, at which the voltage applied on the output-side to the charging capacitor is less than or equal to the reference voltage.
In accordance with the invention, the object relating to the method is achieved by virtue of the fact that on the basis of a charging operation for charging a storage capacitor, the method of operating a capacitor power supply unit with a switching means comprises the steps of:
comparing the phase-sensitive voltage, which is applied on the output-side to a charging capacitor, to a reference voltage,
providing a control signal with a switching time-determining member for the purpose of controlling the switching means, if the applied voltage is less than or equal to the reference voltage,
determining the charging state of the storage capacitor by means of a charging state-determining member,
providing a control signal for the purpose of controlling the switching means, if the charging state of the storage capacitor is greater than a predetermined limit value, which steps relating to the charging state can be performed simultaneously with the steps relating to the switching time,
switching the switching means for the purpose of interrupting the charging procedure of the storage capacitor, if both the charging state of the storage capacitor exceeds a predetermined limit value and also if the voltage, which is applied on the output-side to the charging capacitor, is less than or equal to the reference voltage,
switching the switching means back to the charging operation, if both the charging state of the storage capacitor is less than or equal to a predetermined limit value and also if the voltage, which is applied on the output-side to the charging capacitor, is less than or equal to the reference voltage.
In the case of the capacitor power supply unit in accordance with the invention, the switching time-determining member and the charging state-determining member are electronically connected together by virtue of a logical AND-function and in this manner influence the control input of the switching means. A switching procedure is therefore only triggered, if corresponding control signals from the switching time-determining member and the charging state-determining member are simultaneously applied to the control input of the switching means. The charging state-determining member can supply two control signals, of which one indicates the state of the storage capacitor, wherein the output voltage thereof is greater than a predetermined desired value indicating the limit value of the output voltage, and of which another signal indicates the state of the storage capacitor, wherein its output voltage is less than or equal to the desired value. If the first-mentioned control signal is applied, it is not necessary to recharge the storage capacitor subsequently. If, in contrast, the last-mentioned control signal is applied, then the said storage capacitor is no longer filly charged and can be recharged subsequently. However, the switching means is actually only operated, resulting in the capacitor power supply unit being switched to its charging operation for the purpose of charging the storage capacitor, if the switching time-determining member provides a control signal which guarantees that the switching means is currentless or quasi currentless. For this purpose, the switching time-determining member is influenced by a reference voltage which is extremely low with respect to the mains alternating voltage and expediently is approximately 0 V or equal to 0 V. The control signal of the switching time-determining member is then provided if the voltage applied on the output-side to the charging capacitor is less than or equal to the reference voltage. This ensures that the switching means is only switched if the switching means is currentless or substantially currentless. Furthermore, the logical AND-operation of the two control signals, which influence the control input of the switching means ensures that during permanent operation of the electric power consumer connected to the capacitor power supply unit, the switching means is not switched but remains in its open position, which represents the charging operation of the capacitor power supply unit, until the operation of the electric power consumers has been stopped and correspondingly the output voltage at the storage capacitor has exceeded the desired value as a result of the subsequent charging procedure.
By selecting the switching times and by virtue of the switching procedures which are small in comparison with previously known capacitor power supply units, with regard to electromagnetic compatibility, the capacitor and the mains supply are only subjected to an extremely small pulse loading which can be well below the prescribed limit value.
The capacitor power supply unit in accordance with the invention thus accomplishes the principle of subsequently recharging the storage capacitor in a controlled manner using a soft switching procedure for the purpose of interrupting and switching on the charging procedure. Therefore, even in the case of relatively large, operating point-dependent loads which fluctuate to a considerable extent, the capacitor power supply unit is only heated to an insignificant extent. This type of capacitor power supply unit can thus also be used for the purpose of controlling a relay without having to provide any additional cooling features.
In an expedient manner, the switching time-determining member is provided in the form of a comparator member which serves to compare the applied voltage as a function of the phase of the voltage curve with a predetermined reference voltage. The reference voltage is selected in such a manner that it represents a voltage in the range of the zero crossing of the voltage curve which is applied on the output-side to the charging capacitor. A particularly favourable reference voltage is 0 V, so that it is only possible to actuate the switching means if the switching means is currentless.
It is particularly expedient to provide the elements which are required for the capacitor power supply unit as electronic circuits or switching elements. In an advantageous manner, when providing this type of capacitor power supply unit an electronic semiconductor switch, in particular a thyristor, can be used whose gate has applied to it the outputs of the switching time-determining member and the charging state-determining member. Depending upon the design of the capacitor power supply unit, it can be expedient that connected upstream of the control input of the switching means is a storage member for the purpose of storing the control signals produced by the switching time-determining member and the charging state-determining member. It should be noted in this connection that the use of a thyristor is a possible example and that it is also possible to use different electronic switching means which function in the same manner, such as triacs.
In one capacitor power supply unit having electronic components, the switching time-determining member, which is produced by a circuit, and the charging state-determining member, which is likewise produced by a circuit, are connected together by wires in the form of an AND-function (a so-called wired AND-function). The reference voltage can be provided externally so as to influence the switching time-determining member orxe2x80x94as in a preferred embodimentxe2x80x94said reference voltage can be provided by virtue of a Zener diode which is allocated to the switching time-determining member.